摘要
快速检测铜丝可变形能力、提高铜球可变形性和铜丝热影响区(Heat Affected Zone, HAZ)拉伸强度以及系统研究Cu/Al键合点可靠性问题是铜丝球焊技术应用和发展的关键。本文建立了一种新型、快速、高效的在线测量方法来评估铜丝以及铜球可变形性和铜丝HAZ最大拉伸载荷,并采用该方法研究了不同烧球电流对铜球可变形性、铜丝HAZ最大拉伸载荷的影响。研究了烧球工艺参数对铜球成形质量的影响,对铜球的冷却凝固过程进行了描述。对Cu/Al键合点进行高温老化实验,系统研究了Cu/Al键合界面处金属间化合物(Inter-metallic Compounds, IMC)以及裂纹的生长演变机制,并对IMC成分进行了解析。
在线测量方法对测试环境敏感,金属丝型号、基板类型、劈刀类型等会对在线测量结果产生明显影响,因此在比较不同金属丝样品以及它们对应金属球的可变形性过程中,基板类型、基板温度、劈刀类型以及劈刀的装夹位置须保持一致。在线测量结果表明可变形性高的铜丝未必能生成可变形性高的铜球。
烧球工艺参数对铜球可变形性、铜丝HAZ影响的研究结果表明大烧球电流配合短烧球时间可以提高铜球可变形性,有效降低键合过程中所必需的变形压力;能获得最大拉伸载荷更高、长度更短的HAZ,有利于小外形封装场合中的应用。烧球电流为250 mA时的铜球经0.6 N压力变形后的球高比45 mA下的小14 %,铜球整体可变形性高出7-8 %。250 mA下的铜丝HAZ最大拉伸载荷比45 mA下的高7.5-9.4 %,HAZ长度短22.45 %。
针对于直径为50.8μm的铜丝,当烧球电流在120 mA以上、直径设定值为101.6μm时,铜球在直径、圆度、对称度以及表面质量等几个方面表现最佳。尾丝与打火杆距离(Electrode and Wire Distance, EWD)为0μm时烧球质量最佳。保护气体流量为0.8 l/min时烧球质量最佳,过大保护气体流量将会降低烧球效率。
铜球主要由几个大尺寸柱状晶粒组成。凝固过程开始后,柱状晶粒基于未熔铜丝末端部分晶粒同质外延生长并以辐射状向铜球周围延伸。各柱状晶晶向略有不同,决定柱状晶晶向的主要因素为铜球内部热传递方向。冷却凝固过程中铜球内部热量散失的主要途径为热传导。
可靠性研究结果表明在新形成的Cu/Al键合界面处没有发现IMC。高温老化1小时后键合点边缘区域开始出现Cu/Al IMC。IMC的生长速度随着老化时间的延长而不断下降,IMC层不断横向及纵向生长并延伸至键合点中心位置。在IMC出现之后,IMC层与铜球之间出现了孔洞,萌生位置也为键合点的边缘。随着IMC层的生长孔洞向键合点中心扩展。老化时间超过81小时后,在IMC层与铜球之间形成了贯穿的裂纹,此时IMC停止生长。Cu/Al键合界面间IMC的生长机制为扩散控制反应,其生长行为符合抛物线规律。IMC层主要由三种不同的相组成,分别为CuAl、CuAl2和Cu9Al4,其中CuAl的生成量相对较少。
It has great siginificance to develop a new high efficient method to evaluate the deformabilities of Cu wires. Furthermore, for further application of copper wire bonding, it is well recommended to study the effects of Electrical Flame Off(EFO) process on the Cu Free Air Ball(FAB) deformability and the Cu HAZ breaking strength, and to study the reliability issues of Cu bonds on Al metallization pad. In this thesis, a new on-line measurement is firstly presented to quickly evaluate the deformabilities of Cu wires and Cu FABs with high precision. The effects of EFO process on Cu FAB deformability and Cu HAZ maximum breaking force are investigated using the on-line measurements. We also investigate the effects of EFO parameters on Cu FAB quality and the solidification process during the Cu FAB formation. The growth behaviors of IMCs and cracks in Cu/Al bonds during thermal aging are studied in details. The main phases of the IMCs in the Cu bond are determined using the micro-XRD.
The on-line measurement is found to be sensitive to the bonding conditions. The effects of wire types, substrate types, or capillary types on the measurment result are great pronounced. For a wire or FAB deformability comparison study, it is well recommended that the capillary type, substrate type, and substrate temperature are kept constant and the capillary or its position with respect to the horn should not be changed during the study. It is suggested by the on-line measurement results that the Cu wires with high deformabilities will not produce the Cu FABs with high deformabilities automatically.
During EFO process, the Cu FABs with higher deformabilities which can reduce the impact force for bond deformation during bonding are obtained with a higher EFO current combining a shorter firing time. Furthermore, higher EFO current combining shorter firing time can lead to a stronger and shorter Cu HAZ which is benefical for the wire looping in the application of small scale packaging. Under the deforming force of 0.6 N, the deformed Cu ball height from the FAB with the EFO current of 250 mA is 14 % lower than that from the EFO current of 45 mA. The deformability of Cu FAB from the higher EFO current increases up to 7-8 %. As a consequence, the impact force required for the ball deformation can be decreased up to 7-8 % and the underpad stress can be reduced siginificantly during ball bonding. The Cu HAZ from the EFO current of 250 mA have 7.5-9.4 % higher maximum breaking load and 22.45 % shorter HAZ length compred to that from the EFO current of 45 mA.
For the Cu wire with the diameter of 50.4μm, the optimum Cu FAB can be obtained with the EFO current of 120 mA, the EWD of 0μm, and the FAB preset size of 101.6μm. The flow rate of shielding gas of 0.8 l/min is preferred during Cu FAB formation and the excessive shielding gas will lead to the decrease of the EFO efficiency.
The microstrctural study reveals that the Cu FAB is composed by a few big columnar grains. During solidification, the columnar grains initiates from the unmelted wire end and then grow downwards radially to the free end of the ball. As no nucleation is required, the initial solidification simply adopts and extends the crystal structure of the adjacent solid with which it is in contact and the columnar grain comprises a subset of the solid crystal orientation. The orientation of the columnar grains are mainly controlled by the heat flow direction. During solidification, the principal way to lose heat in the molten ball is conduction through the wire.
In the as-bonded Cu/Al bond, there is no obvious IMC observed at the bonding interface under SEM with the magnification of 1500 times. After 1 h aging at 250°C, the Cu/Al IMC forms at the bond periphery. The IMC grow rate decreases with the increase of the aging time. The IMCs develop laterally and vertically to the bond centre area. Furthermore, some cavities occur between the IMC layers and the Cu bond bottom. With the growth of the IMCs, the cavities start from the ball periphery and grow inward to the bond centre and keep growing to form the cracks as a result. A complete gap between the Cu bond bottom and the upper IMC layer is formed after 81 h aging. The IMCs growth stopes completely as the formation of the cracks. The Cu/Al IMC growth is controlled by diffusion control mode and obeys the parabolic law. The main phases in the Cu/Al IMC layers are confirmed to be Cu9Al4, and CuAl2. CuAl with a smaller amount is believed to be another phase.
引文
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